Allocation of effort in stream food-web studies: the best compromise?

Abstract

The amount and allocation of effort needed to characterize stream food webs was investigated in five replicate streams. Two areas were considered:analysis of community composition (number of individuals sampled)and of diets (number of individuals gutted per animal taxon). Food webs were described by use of consistent methodology, then the effort was retrospectively reduced by considering half of the gut samples (halving dietary analysis effort)and by successively reducing the number of individuals included. Food webs with a reduced number of individuals overestimated connectance and prey:predator ratios, and underestimated species richness, links per species and mean chain lengths. These changes were due to loss of some invertebrate predator species when effort was reduced. In contrast, for dietary analysis the amount of effort expended on non-predatory invertebrates was more influential; halving effort in dietary analysis of non-predatory taxa reduced estimates of connectance and links per species. This study suggests that the effort needed to produce a reasonable estimate is highly dependent on the food-web attribute in question, and that aiming for equity of effort across taxonomic groups is as important as expending greater effort in general.